1. Field of the Invention
The present invention relates to a ceramic heater having a heating element buried within a ceramic body.
2. Prior Art
There is well known ceramic heaters including a ceramic body and a heating member buried within the ceramic body. Such ceramic heaters have been used as heaters for air/fuel ratio controllers, carburetors in the automobile industry, also brazing irons and others. The ceramic body is formed of, for example, alumina or aluminum nitride, taking the form of a cylinder or plate, and the heating member includes, for emitting heat, a heating element made of a high-temperature metal such as tungsten (W), molybdenum (Mo) or rhenium (Re) being buried within the ceramic body and disposed under the outer surface thereof.
The heating element is formed a thin, narrow film in a shape of waved or indented pattern, and both rear ends of the heating element is connected to inner leads which are connected to respective pads provided on the surface of the ceramic body.
For example, for fabricating a cylindrical ceramic heater, as shown in FIG. 17, a green sheet 3 containing a powdery ceramic material is provided with thin metal paste films including a high temperature metal to form a heating member 60 and 6 in a defined pattern on one side of thereof, and is wound on the peripheral surface of a ceramic core 2 or rod so that the paste films makes contact to the surface of the ceramic core 2, and then is fired to sinter the ceramic core 2, green sheet 3 and thin applied film together, the film is formed into a heating member buried under the sintered sheet 3, obtaining a ceramic heater as shown in FIG. 18.
On the green sheet, the paste film includes inner lead portions for connecting the heating resistor to a pair of respective pads, through via holes, which pads are printed on the other side of the green opposite form the paste film. Therefore, in the finally obtained ceramic heater, the pair of pads are provided on the outer surface of the ceramic heater and are connected to the heating resistor through the via holes and the inner lead portions.
Outer leads can usually be fixedly brazed to the respective pads for supplying electric power to the heating resister. The above mentioned blazing method has widely been adopted but as another connecting method it is possible to clamp outer lead parts to the pads.
For brazing the pads, previously plated on the pads is a metallic layer including nickel, chromium or the like, and is brazed with the above mentioned outer leads such as wires made of an Fexe2x80x94Ni alloy or other thermally resistant alloy including Ni and/or Cr.
These ceramic heater have been used for heating sensors such as oxygen sensors for automobiles, in which case, a system is adopted for heating the sensors quickly to start the sensors at the operating temperatures for cold starting, and than the heaters are required to have a high resistance to heat shock, with high endurance against higher temperatures than 800xc2x0 C. High heat emission to shorten the period of time of raising temperature often overrun a defined temperature when cold starting, then giving damage to the sensor electrodes and shortening their useful life time, or reducing endurance of the ceramic heaters.
Prior art discloses a heating member structure for preventing the overheating to speed down the increasing temperature at high temperatures. For example, Japanese Patent Publication No. 8-273813 discloses a ceramic heater wherein a heat pattern is formed in plate-like ceramic body made of Aluminum nitride, the heat pattern being composed of a heating resister, and a further conductive material having a higher temperature coefficient of a resistance than that of the heating resister, which material is connected to the heating resister in series.
Japanese Patent Publication No. 5-34313 discloses a ceramic heater comprising two heating resisters which have different temperature coefficient from each other.
However, it has often been observed that when the ceramic heaters are operated to raise the temperature quickly, for example, at a rate of raising the temperature from room temperature to about 900xc2x0 C. in about 3 second, cracks occur at the surface of the ceramic bodies, sometime the ceramic heaters increasing in electrical resistance.
Further, the structure of brazing the outer leads to the plated metallic layer on the pads, as mentioned previously, have often reduced the brazing strength between the leads and ceramic body. After brazing, as depicted in FIGS. 4 and 5, cracks 91 or cavities have been observed to open in a meniscus of the brazing material layer 9 formed between the outer lead 10 and plated layer 11.
Another problem was to that, where the brazing material layer reached an peripheral edge of the pad on the ceramic, the pad was pealed from the outside of the ceramic heater due to tensile stress applied to the leads, resulting in electrical disconnection to the heating element.
Yet another problem was that the outer leads formed of a Fexe2x80x94Ni alloy tend to increase in grain side during heating, in use, having cracks created in the vicinity of the brazing portion in virtue of metallic fatigue of the brazing material caused by mechanical vibration and/or repeatedly heating.
In the event that a heating element in the ceramic has any defect such as cracks in its segment, the ceramic heater would have a high risk of being superheated at the defect portion, allowing the heater resistance to be further increased so as to lower its useful life.
Further, the environments in which ceramic heaters have been used include circumstances containing high humidity at high temperatures wherein the ceramic heaters must have high endurance. For example, the ceramic heaters using for exhaust gas sensors are subjected to proof-water treatment to prevent the heaters from breaking by heat shock due to water or moisture inserted therein.
However, even in the case where water is prevented from invading directly in the case, actually, water dissolved in air is apt to penetrate into the case along with the ambient air.
Particularly, for Some ceramic heaters including braze layers of metal nickel or a Nixe2x80x94Fe alloy which braze outer leads to the pads and plated layers made of nickel covering the braze layer, oxidative, corrosive elements such as chlorine somewhat remains on the nickel-containing structures.
The residual elements on the structures are changed to create compounds such as NiCl2 or Ni(OH)2 during using in the high-temperature, high-moisture circumstances. Such compounds are concentrated, while deposition and evaporation of water around the compounds are repeated, so as to corrode, through the pitting corrosion of the compounds, the faces of the Ni-based leads and interfaces between the outer lead and the braze layer, or between the braze layer and the Ni-based second plated layer. The corrosion, when further advancing, finally breaks the conductivity of the outer leads to the heating element for power supply.
An object of the present invention is to provide a ceramic heater having a sufficient strength between the outer leads and the pads or the ceramic body.
Another object of the present invention is to provide a ceramic heater having a high thermal-shock resistance when quickly heating to a high temperature.
Another object of the present invention is to provide a ceramic heater being capable of preventing corrosion of the pads and outer leads which have been experience in use.
In the present invention a ceramic heater includes a ceramic body, a heating member buried in the ceramic body and pads disposed on the outside of the ceramic body and electrically connected to the heating member, wherein an outer lead is joined by a braze layer onto a metal plated layer covering each of the pads, and the braze layer contains an amount of 500 ppm or less by weight of palladium.
In the ceramic heater of the present invention, the plated layer is formed on the surface of the metal pads which have been activated with an activating solution containing an amount of 90 ppm or less by weight of Palladium. As a result, the braze layer can not have any cavity in the braze layer.
The braze layer may comprise a metal or an alloy based on gold, copper, or nickel. Any edge of the braze layer secured within an area of and on the pad may be in a minimum distance D of 0.2 mm or more inside from the adjacent edge of the pad to prevent the pad from pealing from the ceramic body when the tensile force acts to the leads against to the ceramic body.
In the ceramic heater of the invention, via holes are each formed in the ceramic body to connect the heating member to the respective pads, and any edges of the braze layers secured on the pads is each in a minimum distance of 0.2 mm or more apart from the respective via holes, to maintain the strength of the leads with respect to the ceramic body.
Further, the outer leads may be formed of nickel or an alloy thereof, and preferably may have a mean crystal grain size of 400 xcexcm or less to maintain the strength of the leads when the tensile force acts to the leads.
In the ceramic heater of the present invention, the heating member includes a heating element containing a plurality of segments disposed in series made of tungsten, molybdenum, or rhenium or an alloy thereof, and any of the segments of the heating element has a notch of a width G of xc2xd or less of the width F of the segment adjacent the notch to prevent the segment from overheat, without breaking the heating element.
The ceramic heater of the invention may be one which is sintered of a ceramic core, a ceramic sheet surrounded integrally onto the round ceramic core, and a heating member disposed integrally between the round ceramic core and the ceramic sheet. Further, the heating element contains a plurality of segments disposed in series to make a pattern made of tungsten, molybdenum, or rhenium or an alloy thereof, and an electric resistance per unit length of the segment at a central portion of the heating element is in a range of 75% to 90% of that of a segment at the peripheral portion thereof.
In the ceramic heater, the heating member may have a heating element encircled around the core, an angle of both edges of the heating element viewing from a center of the core is 90xc2x0 or less.
Further, the pattern of the heating element preferably may have a length in a range of 2.5 to 10.0 mm longitudinally along the ceramic core.
Particularly, a width of the segment at the central portion of the heating element may be in a range of 20% to 90% of that at the peripheral portion of the heating element.
In the ceramic heater of the present invention, a braking element having a higher temperature coefficient of electric resistance than that of the resistance of the segment is connected to the heating member in series, a resistance ratio of the heating member to the braking element is in a range of 1.5 to 10.0, and a length of the braking element is 20 mm or less. In this case, an end of the braking element is connected to pads via inner leads and a distance of the end of the braking element to the pad is 20 mm or more.
A ceramic heater of the present invention include a ceramic body, a heating member buried in the ceramic body and metal pads disposed on the outside of the ceramic body and electrically connected to the heating member, wherein an outer lead is joined by a braze layer onto a metal plated layer covering each of the pads, and a mount of chlorine absorbed on surfaces of the outer leads and the braze layer is 2000 counts or less by measuring in EPMA analysis. It is preferable that the outer leads and the braze layers have previously been heat treated at a temperature of 500xc2x0 C. or more in a reducing atmosphere to facilitate outgassing of chlorine from the metal surface such as braze layer. As a result, the outer leads and/or the braze layer readily can be formed of a nickel or an alloy hereof.